1. Field of the Invention
The present invention relates to an antenna and especially to a universal antenna for receiving satellite signals and terrestrial signals, which is particularly suitable for mobile use. Furthermore, the present invention relates to an antenna modification device for modifying an existing monopole antenna.
2. Description of Prior Art
Quite recently, an increasing number of services have been making use of geostationary satellites for broadcasting radio signals, television signals or other signals. There is a tendency to realize hybrid solutions, i.e. systems in which the signals are broadcast simultaneously via satellite as well as via terrestrial transmitters. By means of this course of action, the advantages of both variants can be combined.
Since, in comparison with signals broadcast by terrestrial transmitters, the received field strengths of satellite signals are much weaker, great store should be set by a maximum antenna gain in the direction of the satellite when the receiving antenna is designed and constructed concretely. In the northern hemisphere the elevation angle between a receiving antenna and geostationary satellites ranges e.g. from 30xc2x0 to 60xc2x0, depending on the parallel of latitude on which the receiving antenna is located. When signals are received, which are broadcast via geostationary satellites, the gain maximum of the receiving antenna should therefore be about 45xc2x0.
Conventional vertical antennas for mobile reception, which are referred to as so-called rod antennas, e.g. the classical xcex/4 monopole, have, due to the nature of the system, a theoretical zero point at an elevation angle of 90xc2x0 and an angle theta of 0xc2x0, respectively. Due to the influence of a finite electric conductivity and losses in the metallic surface area, e.g. the roof of the vehicle, the gain maximum of these antennas is at an elevation angle of from 10xc2x0 to 35xc2x0. When an elevation angle of 35xc2x0 is exceeded, the antenna gain decreases rapidly, whereby the system reserve will be reduced markedly in the case of reception via a satellite. This may, however, result in reception losses. It follows that conventional vehicle antennas are not suitable for receiving terrestrial signals and satellite signals in common.
It is the object of the present invention to provide an antenna for receiving satellite signals and terrestrial signals in the case of which reception losses can be avoided when receiving signals via satellite and to provide an antenna modification device for modifying existing monopole antennas, which permits existing monopole antennas to be modified such that reception losses can be eliminated or reduced when receiving signals via satellite.
According to a first aspect of the present invention this object is achieved by an antenna for receiving satellite signals and terrestrial signals, the antenna comprising a monopole having a first end, which is connected to a feeding point, and a second end. The antenna additionally comprises an unfed dipole which is arranged in spaced relationship with the second end of the monopole and in axial alignment with the monopole and which contributes to the directional pattern of the antenna via a field coupling.
In accordance with preferred embodiments of the present invention, the monopole is a xcex/4 monopole, whereas the dipole has a length in the range from xcex/2xe2x88x9225% to xcex/2+25%. Furthermore, the distance between the monopole and the dipole is preferably smaller than xcex/10. Due to the arrangement of the monopole and of the unfed dipole according to the present invention, the dipole contributes, as a mere passive element, via a field coupling to the resultant directional pattern, i.e. to the far-field pattern, of the antenna. The resultant directional pattern shows a gain maximum in the range of an elevation angle of 45xc2x0. It follows that the antenna according to the present invention is excellently suitable for receiving signals broadcast via geostationary satellites so that reception losses can be eliminated or reduced.
In accordance with a second aspect of the present invention the above object is achieved by an antenna modification device for modifying a monopole antenna having a first end, which is connected to a feeding point, and a second end, the antenna modification device comprising a dipole and a connection element. The connection element serves to connect the dipole and the monopole such that the dipole is unfed and arranged in spaced relationship with the second end of the monopole as well as in axial alignment with the monopole, said dipole contributing to the directional pattern of the antenna via a field coupling.
It follows that the present invention permits existing monopole antennas to be modified so as to be able to realize reception via a satellite without considerable reception losses.